"Shell Oil has crossed another hurdle in its plans to drill three exploratory petroleum wells in the Arctic Ocean.

The Environmental Protection Agency announced Thursday it had approved a clean air permit for Shell to operate its drilling ship in the Chukchi Sea off Alaska's northwest coast.

The permit requires the Frontier Discover to burn ultra low-sulfur diesel fuel. That and other conditions, including restrictions on operating hours, will reduce particulate emissions by 72 percent and sulfur dioxide emissions by 99 percent, from 181 tons per year to 2 tons, said Janis Hastings, deputy director of EPA's air quality program.